Engines may use variable cam timing to vary intake and/or exhaust valve timing to provide various engine operating effects, such as increased fuel economy, increased engine output, increased internal residuals, and more.
Since the above adjustments can affect air-fuel ratio of the combustion cylinders, various approaches may be used to provide fueling adjustments. For example, U.S. Pat. No. 6,729,304 uses the change of cam phase timing between samples to modify the output of the fuel control system. In particular, in this fuel injection control system, when the valve overlap period is changed due to a change in the cam phase, the fuel injection amount is set according to the detected operating conditions of the engine, and then the fuel injection amount thus set is corrected according to the amount of change in the valve overlap period and the wall surface temperature of the intake ports. In this case, the amount of change in the valve overlap period is a parameter in which changes in an internal EGR amount and the amount of intake air due to the change in the valve overlap period are reflected, while the wall surface temperature of the intake ports is a parameter in which a change in the amount of fuel adhering to the wall surface of the intake port, due to the change in the valve overlap period is reflected.
However, the inventors herein have recognized a shortcoming in the above system. If positive valve overlap is used during engine cold starting, especially before intake manifolds and/or intake valves have sufficiently warmed, the residual backflow strips fuel from the port surfaces and back of the intake valves, thereby generating air-fuel combustion errors in the cylinder, even though neither the cylinder air amount nor the intake manifold pressure necessarily correlate with the intake cam timing.
Thus, in one example, in addition to, or in place of, the adjustments of prior approaches, a method is provided for controlling fuel injection into an engine of a vehicle, the engine having a variable valve timing system. The method may include starting engine with the variable valve timing system at a default position; before the engine warms to a warm-up temperature, adjusting the variable valve timing system to increase positive valve overlap of at least one cylinder; and adjusting a fuel injection amount in response to said increase in positive overlap, where said adjustment of fuel provides a decrease in injected fuel to account for an increase in fuel evaporating from the intake valve and/or the intake port as engine temperature warms.
In this way, it is possible to enable a engine to be operated with changes in intake cam timing to allow residual gas from the cylinder to flow back through the intake port at the end of the exhaust stroke (in which the intake valve is starting to open and the exhaust valve is still closing) during engine starting conditions. As such, the above operation may be used as an intended form of exhaust gas recirculation that exploits positive valve overlap to improve pumping losses, reduce NOx formation, and increase fuel evaporation and mixing.
Note that various fuel delivery controls systems may be used that adjust the injected fuel to account for the storage and release of fuel that resides on the intake port and valve surfaces in order to provide the correct amount of fuel in the cylinder. The response of the fuel compensation may be adjusted by estimated manifold pressure and engine coolant temperature.
Also note that the various embodiments herein may be used to augment many different forms of transient fuel compensation.